1. Field of the Invention
The present invention relates to a method for sublimating and purifying an organic material.
2. Description of the Related Art
Organic electroluminescent elements (hereinafter referred to as organic EL elements) have been expected as new self-emission type elements. The organic EL element has a stacked structure in which a carrier transport layer (an electron transport layer or a hole transport layer) and a luminescent layer are formed between a hole injection electrode and an electron injection electrode.
Organic materials are used for the hole transport layers, the luminescent layers, and the electron transport layers. Materials having the property of p-type semiconductors are used for the hole transport layers, and materials having the property of n-type semiconductors are used for the electron transport layers. The luminescent layers also have carrier transportability such as electron transportability or hole transportability, and are composed of organic materials for emitting fluorescence or phosphorescence.
The hole injection electrode, the hole transport layer, the luminescent layer, the electron transport layer, and the electron injection electrode are stacked in this order, thereby forming the organic EL element.
Examples of factors that affect the luminescent characteristics of the organic EL elements include the purity of organic materials. When impurities are contained in the organic materials, the impurities become traps for carriers and cause light extinction, resulting in reduced luminous intensity and luminous efficiency. In order to reduce the amounts of the impurities, therefore, the organic materials must be purified.
As methods of purifying organic materials, recrystallization using solvents and recrystallization by sublimation are generally used. In the recrystallization using solvents, the organic materials can be purified in large amounts. Because the solvents are used, however, the solvents are easily incorporated in organic crystals. The solvents incorporated in the organic crystals function as impurities, which may reduce luminescent characteristics.
On the other hand, in the recrystallization by sublimation, the organic materials are sublimed in the vapor under vacuum and are recrystallized, whereby the impurities are difficult to incorporate. Consequently, the use of sublimation and purification methods prevails for the purification of the organic materials for the organic EL elements.
An article by H. J. Wagner. et al., Journal of Materials Science, 17, 2781, (1982) describes a method of subliming and purifying an organic material. FIG. 3 is a schematic view showing the configuration of a main portion of a sublimation and purification apparatus used for the sublimation and purification method described in the article.
As shown in FIG. 3, a glass tube 1 having a length of approximately 1 m is inserted into a copper tube 2 for thermal conduction. A sample 100 of an organic material to be purified is located near one end in the glass tube 1. A heater 5 is mounted so as to surround the copper tube 2 around the sample 100 of the organic material.
The inside of the glass tube 1 is maintained in a vacuum state of approximately 200 Pa. The heater 5 heats the sample 100 in the glass tube 1, to sublime organic molecules of the sample 100. The glass tube 1 is provided with a temperature gradient, so that organic molecule vapor is cooled near the other end in the glass tube 1, and is recrystallized. Consequently, organic crystals 200 obtained by the recrystallization are formed near the other end in the glass tube 1. After the sublimation of the sample 100 is terminated, the glass tube 1 is broken using a gas burner, to take out the organic crystals 200. The organic material can be thus sublimated and purified.
However, the conventional method of sublimating and purifying an organic material described in the above-mentioned article involves work for breaking the glass tube 1 using the gas burner, whereby the work is complicated. Further, the glass tube 1 must be thrown away after the use, resulting in raised cost.
Furthermore, the organic crystals 200 are easily scattered in the glass tube 1. The organic material is difficult to sublime, and the purification yield thereof is low.